The present invention relates to an antenna array.
A microwave antenna array is widely applied to the field of high-speed scanning radars and microwave imaging. For example, a high-speed scanning radar is applied to radars for detecting flying objects, compact radars, and the like. Microwaving imaging is applied to nondestructive tests, medical diagnoses, temperature imaging enabling low temperature detection, and the like.
The use of a waveguide antenna in a microwave antenna array has been proposed in the prior art. Japanese Laid-Open Patent Publication No. 5-308219 describes a waveguide antenna. In the waveguide antenna described in the publication, a horn antenna is arranged on one side of a dielectric printed circuit.
Known waveguide antennas are described in following documents [1] to [3]:
[1] T. Sehm, A. Lehto, A. V. Raisanen, “A High-Gain 58-GHz Box-Horn Array Antenna with Suppressed Grating Lobes”, IEEE Trans. Antenna Prop., vol. 47, pp. 1125-1130 (1999);
[2] G. M. Rebeitz, D. P. Kasilingam, Y. Guo, P. A. Stimson, D. B. Ruttledge, “Monolithic Millimeter-Wave Two-Dimensional Horn Imaging Arrays”, IEEE Trans. Antenna Prop., vol. 38, pp. 1473-1482 (1990); and
[3] K. Sigfrid Yngvesson et al., “The Tapered Slot Antenna—A New Integrated Element for Millimeter-wave Applications”, IEEE Trans. Microwave Theory Tech., vol. 37, pp. 365-374 (1989).
In the two-dimensional antenna array proposed in document [1], feeder circuit portions are arranged on a single printed substrate, and a horn antenna is arranged on the feeder circuit portions. In the two-dimensional antenna array proposed in document [2], for application to a microwave imaging detector, a thin film including a feeder circuit portion is arranged between a horn antenna and a back cavity. In document [3], an active microwave antenna array including a tapered slot antenna and an active electronic circuit arranged on a substrate is proposed as a two-dimensional millimeter-wave imaging element.
The applicant of the present application has proposed in Japanese Patent Application No. 2008-039009 an active microwave antenna array that arranges Yagi-Uda antennas on a plane. The active microwave antenna array may be applied to microwave imaging reflectometry measurements. A microwave refers to an electromagnetic wave of which frequency is 3 GHz to 300 GHz (one millimeter to ten centimeter in wavelength). The frequency of about 30 GHz to 300 GHz has a wavelength of several millimeters and is also referred to as a millimeter-wave. However, in this specification, microwaves include millimeter-waves.
The prior art structures have the problems described below.
In Japanese Laid-Open Patent Publication No. 5-308219, the horn antenna and waveguide are arranged on one side of the printed circuit substrate, and the horn antenna is arranged on the surface of the dielectric substrate. A feeder (mixer diode) projects perpendicular to the substrate. An intermediate frequency circuit and the like are arranged on the rear surface of the dielectric substrate. Therefore, it is difficult to use active elements, such as mixer diode chips, that are suitable for mass production.
In the antenna array of document [1], only feeders are arranged on the printed circuit substrate, and there is no space for active elements. Thus, the antenna array cannot be used for high-sensitivity imaging receivers.
In the waveguide antenna array of document [2], the space for electronic circuits is extremely small. Thus, to actually lay out electronic circuits, micro-fabrication techniques for fabricating semiconductor integrated circuits are required.
The tapered slot antenna may be used for a wide band. However, each of the waveguide antennas are large. Thus, when a large number of waveguide antennas are arranged to form an imaging element, the spatial resolution becomes low.
The planar Yagi-Uda antenna proposed by the applicant of the present application has a satisfactory spatial resolution. However, in the array structure, interference between adjacent antenna elements occurs and forms deep notch in the frequency characteristics. Thus, the planar Yagi-Uda antenna is not suitable for a wide band antenna that performs frequency sweeping. Further, the printed circuit substrate is thin and lacks mechanical strength.